[A-HW33] Prediction of water and sediment dynamics from small to large scales

[AHW33-P05] Factors controlling concentration of dissolved ⁹⁰Sr and ¹³⁷Cs in river water in the Chernobyl exclusion zone.

*Matsushita Koki¹, Yasunori Igarashi², Yuichi Onda¹, Yoshifumi Wakiyama², Alexei Konoplev², Gennady Laptev³, Sergii Obrizan⁴, Volodymyr Demianovych⁴, Mark Zheleznyak² (1.Tsukuba University, 2.Institute of Environmental Radioactivity at Fukushima University, 3.Ukrainian Hydrometeorological Institute, 4.Chernobyl Ecocentre, State Agency of Ukraine on Exclusion Zone Management)

Keywords:Chernobyl catchment, Contaminant transport, Strontium-90 and Cesium-137, Wetland, Spatial-temporal variations

Dynamics of radionuclides, ⁹⁰Sr and ¹³⁷Cs, are still of great interest for mitigating radiological risks in the Chernobyl Exclusion Zone (CEZ) and its downstream area. Previous studies showed positive relationship between dissolved ⁹⁰Sr concentration and water discharge and it suggest that hydrological characteristics affects the ⁹⁰Sr transfer. However, in practice, the dynamics of radionuclides is closely related not only to hydrological characteristics but also to chemical characteristics and the surrounding environment. For understanding formation of solution concentration in the river water system, several studies attempted to evaluate spatial-temporal variations of factors of interest in a catchment and discussed relationship between catchment characteristics and the behavior of the factors. Such approaches can improve our understanding of radionuclides dynamics. The aim of this study is to clarify the factors controlling dynamics of radionuclides in the river water system in the CEZ and we evaluated spatial-temporal variations in dissolved ⁹⁰Sr and ¹³⁷Cs concentration in a small catchment. The study site was in the Sakhan catchment, locating at approximately 7 km northwest from the Chernobyl Nuclear Power Plant. Water samples were taken at main stream, tributaries and water spring points each season. The discharge rate was determined by cross-sectional survey of flow velocity and water level at the same time as water sampling. Each catchment area was made from digital elevation model (DEM) data and mean ⁹⁰Sr and ¹³⁷Cs inventory for each sub-catchment was determined by distribution maps of radionuclide. Moreover, the proportion of wetland in dry and wet season was calculated from satellite spectral images. In laboratory, samples were analyzed for dissolved ⁹⁰Sr and ¹³⁷Cs, dissolved anions and cations and Si concentration. Discharge rate at the main stream was the highest in snowmelt season and the lowest in summer. The concentration of dissolved ⁹⁰Sr and ¹³⁷Cs at the main stream was, as like discharge rate, high in snowmelt season and low in summer. However, the response of dissolved ⁹⁰Sr concentration to discharge rate was not clear at the spring points and river water in some headwater catchments. We also found that the positive relationship between the area of wetland and dissolved ⁹⁰Sr and ¹³⁷Cs concentration in the snowmelt season. Previous studies have showed that “wetlands” and “high water level in the floodplain” can affect high ⁹⁰Sr discharge. However, the relationship is not clear during the summer. From these results, the tight coupling between hydrological processes and the ⁹⁰Sr and ¹³⁷Cs concentration formation processes is suggested, and further study is necessary to elucidate the mechanisms that control the ⁹⁰Sr and ¹³⁷Cs concentration in the river water systems.